Miniature resonant photonic devices as known in the art are created from coupled high Q-factor cavities (e.g., ring resonators, photonic crystal resonators or the like). The resonance is a result of circulating whispering gallery modes (WGMs) that are created within circular structures (such as around the circumference of an optical fiber), where the WGMs traveling around the circumference of the structure undergo repeated internal reflections at near-grazing incidence. The leakage of light can be very small in these structures, leading to high intrinsic quality factors (Q factors). The Q factor is generally defined as a measure of energy loss relative to the energy stored in a resonator (or any type of oscillating device), and can be characterized by the center frequency of a resonator divided by its bandwidth (a common value for a “high Q” resonator is a value on the order of 109 or more). The preferred “high Q” resonator is therefore associated with a relatively narrow and sharp-peaked resonance feature.
Conventional resonator structures are formed by creating features whose size is of the order of the wavelength of the propagating optical signal, or greater. For example, known rings or toroids or spheres are typically tens of microns in dimension. Such structures are commonly created using lithographic techniques (for example, etching a silicon material to create the feature pattern) with the undesirable result of surface roughness. The lithography-associated roughness leads to scattering of a propagating optical signal, reducing the Q factor of the device. In addition, the inaccuracies of the conventional fabrication process limit the precision with which multiple devices can be coupled together to form more complex structures. While it would be useful to create resonators with even smaller dimensions (i.e., sub-wavelength), which offers certain advantages in terms of performance, such smaller dimensions pose additional difficulties in fabrication.
Previously, we have developed various complex, coupled photonic microdevices within and along an optical fiber, using sub-wavelength-sized perturbations of the fiber's radius to create resonance cavities. Multiple microstructures may be formed along a given length of optical fiber and coupled together to create complex photonic microdevices. Details of this device structure can be found in commonly-assigned US Publication 2012/0213474, dated Aug. 23, 2012 and hereby incorporated by reference.
However, when attempting to create relatively long chains of these devices, fabrication errors begin to impair their performance, with the errors growing with the length of the chain. One source of error may be nanometer-scale non-uniformities in radius of the fiber, which may then continue in cumulative fashion to affect all devices along the chain. Other sources of fabrication error in creating long chains of microresonators include, but are not limited to, surface contamination of the fiber, imperfections in system alignment (i.e., the system used to create the effective radius variations in the first instance), fluctuations of the beam power used to create effective radius variations, non-uniform doping profiles in photosensitive fibers and the like.